In a GSM (Global System for Mobile communications) system, a mobile station (MS) making a voice call tracks a radio link counter, which is used to ensure the quality of the radio link (see 3GPP TS 45.008 Section 5.2). The radio link counter is used to measure the quality on the Slow Associated Control Channel (SACCH) associated with a connection (which may be used to carry a voice call). The SACCH is defined in 3GPP TS 45.002. At the start of a call, after handover, and after re-assignment, the radio link counter “S” is initialized to a network-defined Radio Link Timeout (RLT) value. After every bad SACCH block, S is decreased by 1. After every good SACCH block, S is increased by 2 (to a maximum value of RLT). If the radio channel conditions are bad, many radio blocks will be lost, and eventually the radio link counter will expire when the value of S equals the expiry value (zero). This event is termed Radio Link Failure (RLF), and at that point the device stops using the traffic channel.
The following are some commonly-seen problems leading to radio link failure:    a) Rapid radio channel degradation (e.g., due to sudden co-channel interference, an MS operating near the sensitivity limits, etc). In this case, the network may try to send a handover command to the MS, but the MS will not be able to decode and act on it. Thus, it will be “stuck” in the cell until RLF occurs.    b) The network not sending a handover message in time to avoid RLF. This could be because the handover target cell is too congested to support another call.    c) Uplink interference and/or limit-of-sensitivity (due to limited transmit power) issues. In this case, even if the MS can receive the downlink signalling, the network may not be able to decode the MS transmissions, which will result in the MS being unable to handover successfully (for example, because measurement reports from the MS are not decoded).
In GSM cells that have an enabled call re-establishment (CRE) feature (see, for example, 3GPP TS 24.008 Section 4.5.1.6, 3GPP TS 45.008 Section 6.7.2), there is a method for the MS to recover from RLF. If the mobile station is not able to access any cell which has CRE enabled, RLF results in a call drop.
If the mobile station is able to access one or more cells that have CRE enabled, a MS can use a CRE procedure, that involves finding the best cell (either a new cell, or the current cell) having an enabled call re-establishment feature, by acquiring the relevant system information (SI) messages; sending a message on the Random Access Channel (RACH) requesting a call re-establishment; and then after receiving an immediate assignment, sending a CM (connection management) Re-establishment Request message.
Successful call re-establishment allows the device to re-establish the traffic channel and to continue the call without interruption (the user does not need to manually re-dial the call and wait for the other party to answer).
An existing approach to attempting call re-establishment involves selecting the target cell, then completing an Immediate Assignment procedure. This involves repeating the process of {sending a request; waiting a random time} up to MAX_RETRANS times, and then starting a 5 second timer to ascertain that no response is coming after the last channel request message is sent. (Spacing between Channel Request messages is specified by the algorithm in 44.018 Immediate Assignment Procedure). The same procedure can be started on a subsequent cell, but not before the entire procedure (including all MAX_RETRANS iterations on the first cell, and the 5 second timeout) has been completed in the previous one.
In some cases, a mobile station will not attempt call re-establishment, even if RLF has occurred and the mobile is able to access one or more cells with CRE enabled. This is dependent on the use of the connection between the MS and the network. If the mobile station does not have an ongoing call, or all ongoing calls are in the in the call establishment or call clearing phase, i.e. any state other than the “active” state or the “mobile originating modify” state (see 3GPP TS 24.008 v.8.5.0) then call re-establishment will not be attempted, and the connection will be terminated.